We recommend periodic RF system testing to reduce the potential for major burnouts (have a look at what can go wrong versus catching an issue early). When the system should be swept, how often and the recommended measurements will vary with the size of the system, its configuration, power capacity, environmental conditions, and other factors. We can help identify an appropriate testing schedule on a case by case basis. Customers that participate in our scheduled maintenance program already have the appropriate RF system measurements performed on a routine basis. April 2014 Update with added thoughts from conversations at the NAB Show Including why you shoud have your system swept now.
Regardless of system configuration, it is essential to have reference data on hand at the transmitter site. Whenever a new system is installed, or changes or repairs are made, a complete set of reference data should be recorded. The reference data should clearly identify the sweep start and stop frequencies for both time and frequency domain plots, the number of points used, where and how the connection to the system was made, notations of any anomalies. Photos of the system can also be very helpful. Proper documentation will include sufficient data to allow the test conditions to be duplicated, which ultimately makes it easier to identify changes in the system. Here's a brief list of recommendations.
What constitutes complete data varies between RF system manufacturers, as well as between technicians and engineers. However it is safe to identify some basic requirements:
Transmission lines and antennas: Generally, frequency domain data across the assigned bandwidth and twice the assigned bandwidth should be presented in polar, VSWR and Return Loss formats. Time domain data for spans across the assigned bandwidth, twice the assigned bandwidth, across 100 MHz, centered on the assigned channel, and a broadband sweep of up to 400 MHz. The broadband sweep parameters (center frequency and span) vary based on the size of line, its length and the termination type. For transmissioni line runs in excess of 1 µSec, the plots are broken into 1 µSec or smaller segments to improve detail. Markers identify the antenna input, the elbow at the tower base, the gas barrier, any anomalies or components of interest that may appear. Frequency domain markers vary with service type.
Filters and combiners: Here again, frequency domain data across the assigned bandwidth and at least twice the assigned bandwidth should be recorded (I typically adjust the two times bandwidth to a meaningful value; i.e. a 6 MHz TV channel would be swept at 20 MHz rather than 12 MHz so the span is 2 MHz rather than 1.2 Mhz per division). S21 measurements showing response in a log mag format at 10 dB per division, insertion loss in a log mag format at .1 dB per division, isolation and group delay should be scaled as appropriate are recorded. S11 measurements are plotted in the same fashion as with lines and antennas. Some addional plots may be recorded as appropriate.
Of course, verifying proper pressurization, building temperatures and other mechanical parameters are addressed in our periodic maintenance visits.
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